The objective of this proposal is the complete characterization of the photoreactions of DNA "in vivo" and "in vitro" with the psoralens. These compounds have been reported to be mutagens and carcinogens and they are used clinically in the treatment of skin pigmentation disorders and skin cancer. The discovery in our laboratory that DNA in chromatin and intact nuclei is protected from the photoinduced cross-linking reaction of the psoralens and that this protection is relaxed approximately every 180 base pairs indicates that DNA in chromatin subunits or nucleosomes is protected while the DNA between subunits is not, a situation identical with that observed with nuclease digestion. We propose to exploit this observation in the study of DNA structure in chromatin, metaphase chromasomes, DNA satellite sequences, SV-40 during lytic infection, E. coli cells, spheroplasts, and Worcel particles, and in the superinfected lambda phage system. Our main instrument of observation is the electron microscope where DNA, heat denatured, is spread under denaturing conditions. DNA fractionation by density gradient techniques is still effective after psoralen cross-linkage, as is superhelical structure analysis. We believe this new discovery provides a unique method for studying uniformity and variability of protein-DNA structures "in vivo".